Meet the weird amphibian that rules the underworld

HERE’S what you need when you go in search of the world’s largest and fiercest cave animal: a hard hat, wellies, a torch, a bottle of schnapps and a lot of patience.

One thing you don’t need is anything to defend yourself with. The world’s largest and fiercest cave animal is neither very large nor very fierce. Growing to about 25 centimetres long, they are pale, slender and clammy with short, feeble legs, much like a real-life Gollum. They are also virtually blind. So you are more likely die of boredom or cold than come under attack. But what they lack in ferocity, they make up for in strangeness.

I have come to Postojna in Slovenia in search of the olm, Proteus anguinus – a rare white salamander that ekes out a slow, silent and very long life in the caves of the western Balkans. Known locally as Človeška ribica, or “human fish”, because of their pinkish skin and aquatic lifestyle, olms were once only known from specimens washed out of caves by flooding; legend had it they were baby dragons. Now they are Slovenia’s national animal. When the country became independent after centuries of French, Austro-Hungarian, Italian, German and Yugoslavian rule, it gave olms pride of place on its coins.

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Postojna looks like the sort of nondescript town you would pass through on the way to somewhere more interesting. But beneath the surface it is breathtakingly beautiful. It sits on the edge of the Dinaric karst, a landscape of limestone and dolomite covering much of the former Yugoslavia. Karsts are made of porous, soluble rock and are famous for their spectacular cave systems, carved over millions of years by underground rivers and embellished by the steady drip, drip, drip of water percolating through the rocks above.

Slovenia has something like 10,000 karst caves, but the one at Postojna is the largest (see diagram). It has been a tourist magnet for 200 years, with around 5 kilometres open to the public. But behind the scenes there’s a lot more – around 24 kilometres in total, much of it still unexplored. A 3.5-kilometre section was discovered earlier this year. These quiet, largely unlit sections are the lair of the olm.

Foot-long megafauna

Our guide takes us through a little-used back entrance at the bottom of a deep sinkhole in the forest. Thus begins a 5-hour trek through some of the most spectacular subterranean scenery in the world, a glorious parade of cathedral-sized caverns and narrow passages festooned with stalactites, stalagmites and flowstones. There are also networks of tunnels blasted by the Italian army between the wars, when the cave passed under the border between fascist Italy and the Kingdom of Yugoslavia. Mussolini visited the cave system in the 1930s and was presumably impressed by its extremely punctual underground railway.

Geology and history aside, we are on the lookout for olms, though chances are slim. Accustomed to darkness and silence, they are spooked by light and noise, and vanish when people approach.

For hours, the only living things we see are bacterial colonies on the walls and the odd cave cricket. Then we try our luck in a low-ceilinged side tunnel that is too young to have accumulated any large stalactites. The mud is knee-deep, the air dank and cold. We pause and drink schnapps, for luck and warmth, and then slowly and quietly approach a shallow pool known to be an olm hangout.

You don’t tend to think of small, pallid salamanders as charismatic megafauna, but in cave terms that is exactly what they are. In this ecosystem they are the apex predator, devouring the equally pale and blind crustaceans that share the darkness with them. There’s only one thing that eats olms – other olms (and occasional river fish that get lost in the cave after floods). The males are ferociously territorial.

Nonetheless, life down here is far from action-packed. With no sunlight to fuel photosynthesis the cave ecosystem is extremely low-energy; the only input is organic matter leaching in from the outside world, plus the meagre contribution of bacteria that get their energy by breaking down inorganic molecules. Olms have evolved to go long periods without food – in captivity they are fed just three tiny cave shrimps a day, and can survive quite happily for 12 years without eating anything at all.

The near-starvation diet, cold environment and low metabolic rate may go some way to explaining the olm’s extraordinary longevity: they are estimated to live about 80 years on average and may get past 100, a remarkable innings for such a small animal. Olms have long been of interest to gerontologists keen to discover the secrets of eternal youth.

They are biological curios for other reasons, too. Their eyes are vestigial and they lack pigmentation, both classic adaptations to cave life. They are neotenous, meaning they don’t metamorphose into an adult form, retaining juvenile characteristics including external gills and the ability to regenerate their limbs and tails (early experiments trying to force them to grow up failed). Unlike most other salamanders, they are entirely aquatic.

Their ecosystem is also remarkable, the Amazon of the energy-starved underworld. “We have the richest underground biodiversity in the world,” says Tadej Slabe, head of the Karst Research Institute in Postojna. That includes crickets, spiders, pseudoscorpions, snails, millipedes and beetles. Among those are the slender necked beetle, the discovery of which in 1831 kick-started the study of cave-dwelling creatures, known as speleobiology, and Anophthalmus hitleri, discovered in 1933 and named after Hitler (fascist connections are never far away in this part of the world). All are true troglobites, which means they are unable to survive outside the caves.

Olms have been studied for the best part of a century, but there is much that remains unknown. They are presumed endangered thanks to their rarity and the presence of threats such as pollution and groundwater extraction, and they are protected. But assessing their numbers is challenging. The standard tool for estimating population size, capture-mark-recapture, is not much use: if you mark an olm by making a nick in its tail or cutting off a toe, the method typically used for amphibians, it just grows back.

So biologists at the Tular Cave Laboratory – another Slovenian karst cave, and a speleobiology lab since 1960 – are pioneering a new technique. Like many aquatic animals, olms leave traces of DNA in the water which can be collected and amplified, revealing their presence and allowing scientists to estimate their numbers even if the ghostly creatures are nowhere to be seen.

Environmental DNA will also help determine whether olms are a single species, or many. One population of olms, discovered in a cave in south-eastern Slovenia in 1986, have black skin and functioning eyes. They are classed as a subspecies but may deserve separate status, which could also be true of other isolated populations. Early results suggest that there could be at least three species, each perhaps numbering no more than a few thousand individuals.

Postojna cave may be home to as few as 200 olms, which makes our mission seem even more quixotic. But at the end of our squelchy slog we are rewarded with a rare glimpse of three of them. They glide sinuously through the water in front of us before swimming away to escape the glare of our torches. Out comes the schnapps, this time in celebration, and we turn back towards warmth and light, leaving the darkness to the creatures that own it.

(Images: Blickwinkel/Alamy Stock Photo, Stuart Butler)

Discover more troglobites in the gallery below:

This article appeared in print under the headline “Crouching caver, hidden dragon”